1. Field of the Invention
The present invention relates to a tunable thin film capacitor with a thin film dielectric layer whose dielectric constant is changed by the voltage applied across a lower electrode layer and an upper electrode layer.
2. Description of the Related Art
It is so far known that a strontium titanate (SrTiO3) thin film that is a paraelectric substance or a strontium barium titanate (Ba, Sr) TiO3 thin film that is a ferroelectric substance exhibits a non-linear change in the dielectric constant thereof upon application of a predetermined voltage to the dielectric thin film thereof (A. Walkenhorst et al., Appl. Phys. Lett. 60 (1992) 1744 and Cem Bascri et. al., J. Appl. Phys 82 (1997) 2497).
Thin film capacitors using as their thin film dielectric layers perovskite structure ferroelectric oxide thin films of these strontium titanate and strontium barium titanate, etc. are proposed (Japanese Unexamined Patent Publication JP-A 11-260667 (1999)).
These tunable thin film capacitors are each constituted in such a manner that, on a support substrate 161, a lower electrode layer 162, a thin film dielectric layer 163 and an upper electrode layer 164 are successively formed by deposition as shown in a sectional view of FIG. 24. More specifically, on the support substrate 161, a conductor layer that will become the lower electrode layer 162 is formed by deposition over approximately the whole surface of the substrate 161, and then, patterning is performed to form the lower electrode layer 162 of a predetermined shape. Next, the dielectric layer 163 is formed on the lower electrode layer 162. This dielectric layer 163 is formed, by the thin film method, with a mask placed at a predetermined position, or formed by the spin coating process, and then patterned into a predetermined shape. Further, the layer is heated for hardening as required. The upper electrode layer 164 is formed in such a manner that a conductor layer that will become the upper electrode layer 164 is formed on the dielectric film 163 and then pattern-processed. Here, the facing region of the dielectric layer 163 which is actually held between the lower electrode layer 162 and the upper electrode layer 164 becomes a capacitance-producing region.
In the dielectric layer 163 in this capacitance-producing region, the dielectric constant that the dielectric layer 163 has is changed by an external control voltage fed across the lower electrode layer 162 and the upper electrode layer 164.
Accordingly, in case that the mutually facing areas of both electrodes layers 162 and 164 and the thickness of the dielectric layer 163 are made constant, the capacitance value obtained between both electrode layers 162 and 164 can be varied by setting the external control voltage at a predetermined voltage.
Further, the thin film capacitor shown in FIG. 24 is constituted such that, in order to prevent the lower electrode layer 162 and the upper electrode layer 164 from short-circuiting to each other, the upper electrode layer 164 is extended onto the support substrate 161 so as to form an air bridge 165. In other words, by extending the upper electrode layer 164 onto the support substrate 161, a space is formed around the dielectric layer 163.
This space can be formed in such a manner that an organic resist member that is formed by heat treatment or the like is formed, and thereafter, the upper electrode layer 164 is formed and heat-treated.
In such a variable capacitor, the capacitance thereof is changed by a control voltage applied across the lower electrode layer 162 and the upper electrode layer 164, and therefore, it is important that this voltage be uniformly applied to the dielectric layer 163.
This external control voltage is, for example, ten to several tens of volts, but this external control voltage is, actually, hard to be uniformly applied to the capacitance-producing region; and thus, it is difficult to stabilize the dielectric constant of the dielectric layer 163 to a predetermined value. For example, by taking the maximum capacitance into consideration, the facing area of the capacitance-producing region is set, but, in case that an attempt is made to obtain this capacitance value from one capacitance-producing region, the area of the capacitance-producing region is increased. As a result, the voltage drop of the control voltage occurs in the upper electrode layer 164 and the lower electrode layer 162; and, of the lower electrode layer 162 and the upper electrode layer 164, the portions adjacent to portions 162a, 164a extended from the capacitance-producing region become high in potential, while, in the central portion of the capacitance-producing region and portions away from the extension portions 162a, 164a shown in FIG. 25, the potential becomes relatively low. In other words, in the same capacitance-producing region, a distribution of potential takes place, resulting in the problem that the sufficient dielectric constant control, i.e., a sufficiently wide variable range, cannot be obtained; due to this, there has been the problem that a capacitance corresponding to the external voltage cannot be obtained as according to the specifications.
Further, in order to structurally prevent the upper electrode layer 164 and the lower electrode layer 162 from short-circuiting to each other, the air bridge 165 is provided. The existence of this hollow structure results in giving a large restriction when the capacitor is mounted on a mother board and in lacking in reliability. Further, as for the method of manufacturing the upper electrode layer 164, an organic resist is used for the formation of the air bridge 165, and thus, a manufacturing method deviating from the thin film technology must be used, so that the manufacturing steps become very complicated.
To a tunable thin film capacitor that operates in a high-frequency region, it is important to reduce the loss of the respective electrode layers. For this, it necessary to increase the thickness of a lower electrode layer 162 and an upper electrode layer 164. In practice, however, it is difficult to increase the thickness of the lower electrode layer 162.
It is because, in the case of increasing the thickness of the lower electrode layer 162, the coverage of the lower electrode layer 162 to the dielectric layer 163 is deteriorated. Further, separation by peeling takes place between a substrate 161 and the lower electrode layer 162. Moreover, separation by peeling takes place between the dielectric layer 163 and the lower electrode layer 162. These inconveniences are caused because, by increasing the thickness of the lower electrode layer 162, the stress resulting from the difference between the thermal expansion coefficients of the substrate 161 and the dielectric layer 163 is increased.
Particularly, in the case of such a variable capacitance element, the increase in thickness of both electrode layers 162, 164 is very important in view of the fact that a relatively high capacitance controlling voltage of about 10 V must be applied to the dielectric layer 163 between the lower electrode layer 162 and the upper electrode layer 164.
Further, in the case of the known tunable thin film capacitor, an air bridge 165 was formed around the dielectric layer 163 in order to prevent the upper electrode layer 164 from short-circuiting to the lower electrode layer 162. Thus, a portion of the upper electrode layer 164 is positioned in the air like a bonding air, so that it is very difficult to actually mount the capacitor onto a mother board or the like; and thus, the known tunable thin film capacitor could hardly be put to practical use.
Concerning a tunable thin film capacitor constituted such that, by application of an external voltage, the dielectric constant of the dielectric layer is changed to vary the capacitance, the important problems to be solved are to reduce the voltage loss and signal component loss (hereinafter referred to merely as loss) due to the resistance component of the electrode portions and to improve the drapeability or coverage of the dielectric layer.
However, in the case of the known structure, if the thickness of a lower electrode layer 162 is increased in order to reduce the loss of the lower electrode layer 162 and the upper electrode layer 164, the coverage of a dielectric layer 163 for the lower electrode layer 162 is deteriorated. Further, when an external voltage (driving voltage) for varying the capacitance is applied, the electric field intensity becomes the maximum in a thin portion of the dielectric layer 163 (for example, the end of that portion of the dielectric layer 163 which covers the lower electrode layer 162), whereby dielectric breakdown takes place. Or, a crack is made in the dielectric layer 163, so that the lower electrode layer 162 and an upper electrode layer 164 opposite to it are short-circuited through the dielectric layer 163.
Further, there is the problem that, in case that a crack is produced at the end of the dielectric layer 163 in the externally extended portion of the upper electrode layer 164, of the lower electrode layer 162 and the upper electrode layer 164 that face each other through the dielectric layer 163, the portion that constitutes the capacitance is opened, and thus, the designed capacitance cannot be obtained.
Among the capacitors, there is a thin film capacitor constituted such that the electrode layers and the dielectric layer being constituent elements of the capacitor are formed of thin films. In this capacitor, normally, the thin film lower electrode layer, the thin film dielectric layer, and the upper electrode layer are successively laminated on an electrically insulating substrate in this order. In such a thin film capacitor, the lower electrode layer and the upper electrode layer are respectively formed by sputtering, vapor deposition or the like, and in addition, the dielectric layer is also formed by sputtering, the sol-gel process or the like. In the manufacture of such a thin film capacitor, normally the photolithography method is employed as follows: First, a conductor layer that will become the lower electrode layer is formed over the whole surface of an insulating support substrate, only the necessary portion of the conductor layer is then covered with a resist, and thereafter, the unnecessary portion of this conductor layer is removed by wet etching or dry etching to form the lower electrode layer of a predetermined shape. Next, on the support substrate, a dielectric layer that will become the thin film dielectric layer is formed over the whole surface, and the unnecessary portion of this dielectric layer is removed therefrom as in the case of the lower electrode layer, whereby the thin film dielectric layer of a predetermined shape is formed. Finally, a conductor layer that will become the upper electrode layer is formed over the whole surface, and the unnecessary portion of this conductor layer is removed to form the upper electrode layer of a predetermined shape. Further, a protective layer and solder bumps are formed, whereby it becomes possible to surface-mount the capacitor. Further, a tunable thin film capacitor constituted such that, as the material of the thin film dielectric layer, a dielectric material comprised of (BaxSr1−x) TiO3 is used, and, by applying a predetermined potential across the upper electrode layer and the lower electrode layer to change the dielectric constant of the dielectric layer, whereby the capacitance is controlled, is also of the same structure.
In order to use a thin film capacitor as, e.g., a capacitor in a high-frequency circuit, the self-resonant frequency thereof must be positioned at the side higher than the frequency used. Such a thin film capacitor can be realized by making the inductance at the lower electrode layer and the upper electrode layer small; a thin film capacitor with a small inductance is disclosed in, e.g., Japanese Unexamined Patent Publication JP-A 8-241830 (1996).
In order to use a thin film capacitor as a capacitor in a high-frequency circuit as mentioned above, the self-resonant frequency must be positioned at the side higher than the used frequency, the inductance must be small, and in addition, the loss of the lower electrode layer and the upper electrode layer must also be low. This is because, even in case that the resonance point is positioned at the side higher than the frequency at which the capacitor is used, the impedance resulting from the capacitor is small at a frequency in the vicinity of the resonance point, so that, in a capacitor with a large loss, the resistance component becomes predominant. Therefore, in order to reduce the loss due to the lower electrode layer and the upper electrode layer, a metal with a small resistivity must be used as the material of the lower electrode layer and the upper electrode layer, and the lower electrode layer and the upper electrode layer must be made as thick as possible.
Further, by making the capacitance of the capacitor small, it becomes possible to shift the self-resonant frequency thereof further towards the high-frequency side, and thus, the increase in the loss by the influence of resonance can be reduced. In order to reduce the capacitance of the capacitor, it is necessary to decrease the area of the capacitance-producing region comprised of the thin film dielectric layer sandwiched between the lower electrode layer and the upper electrode layer, but, by decreasing the area of the capacitor, a step that deteriorates the leakage characteristic is formed in the dielectric portion constituting the capacitor, and the accuracy in positional matching becomes hard to achieve at the manufacturing steps, thus resulting in a fall in manufacturing yield.
In order to use a thin film capacitor as a constituent part of a filter, a resonator or the like in a high-frequency circuit, it is required for the capacitor to have a high Q-value. The Q-value depends on the losses in the respective constituent elements of the capacitor; the dielectric loss of the dielectric, the ohmic loss in the electrodes, etc. are the main causes for lowering the Q-value.
In order to reduce the loss due to the lower electrode layer and the upper electrode layer, it is necessary to use a metal with a low resistivity; gold, silver, copper, aluminum, etc. are efficient, but, in the case of these metals, there is the possibility that there may be caused the problem that these metals are each oxidized at a high temperature during the manufacturing steps, the film quality thereof is deteriorated, the metals are chemically reacted with the dielectric layer. For example, in the case of an electrode comprised of gold, the smoothness of the electrode is deteriorated at a high temperature, which will cause the short-circuiting of the capacitor. Due to this, the structure constituted such that, on a layer of tungsten or molybdenum that is a high-melting metal material, platinum or the like is laminated as a reaction-preventing layer has so far been used as an electrode material (See Japanese Unexamined Patent Publication JP-A 9-260603 (1997)).
Further, it is a common practice to laminate an adherent layer using Ti or Cr between the support substrate and the electrode material for the purpose of enhancing the adherence.
However, there has been caused the problem that the high-melting metal material is low in conductivity as compared with gold or the like, and, in particular, the electrode loss cannot be reduced in a high frequency region higher than 1 GHz. Further, in case that platinum is used as the material of the reaction-preventing layer, there has also been caused the problem that the thermal stress of the reaction-preventing layer is large, so that the reaction-preventing layer is apt to peel off from the interface between the lower electrode layer and the support substrate due to the change in temperature during the manufacturing steps. Further, in case that gold is used as the conductive layer of the lower electrode layer, the film quality is markedly deteriorated due to the high temperature during the manufacturing steps, and thus, there have been caused problems such as the problem that the capacitor is short-circuited and the problem that the leakage current becomes large. Further, Ti, Cr, and the like are easy to react with BST and PZT that are general dielectric materials, so that, in case that these metals are used as adherent layers, there is the possibility that the characteristic of the dielectric may be deteriorated at the time of a process using a high temperature.
In such a tunable thin film capacitor, the upper electrode layer 164 was made into an air bridge structure 165, and therefore, the electrode length thereof became large, so that the conductor loss became increased. Further, due to the curved shape which is peculiar to the air bridge structure, there was caused the problem that the self-inductance L was large, so that the self-resonant frequency, defined by the condition: fo=½ (LC)1/2 wherein C represents a static capacitance, was small, and thus, the operating frequency was limited to a low frequency region.
As a variable capacitance capacitor, there has so far been used a varactor diode constituted such that, by applying a reverse bias to the diode, the capacitance is changed. A diode is used in a rectifier circuit, etc., utilizing the phenomenon that, when a bias is forwardly applied to the PN junction thereof, a current flows. In the PN junction interface, there exists a region called a depletion layer in which no electron or hole exists. When a reverse bias is applied to the diode, the electrons and holes are both pulled in the direction away from the PN junction interface, so that the depletion layer becomes thick, and the thickness of the depletion layer changes depending on the magnitude of the reverse bias. This depletion layer can be considered to be a dielectric, so that, in case that a reverse bias is applied to the diode, the thickness of the dielectric changes depending on the magnitude of the reverse bias, as a result of which the diode can be utilized as a capacitor whose capacitance changes.
The varactor diode is standardized for the utilization thereof as a variable capacitance capacitor. The varactor diode is used as the variable capacitance capacitor in communication apparatus, but, recently, due to the increase in the demand for portable communication terminals, the frequency band used for communications is being made higher, and in addition, efforts are being made to lower the voltage at the terminals. In the varactor diode, its loss is increased at high frequencies, and, in the case of using a low voltage, the depletion layer becomes thin, and the leakage current increases; and thus, the varactor diode theoretically ceases to function as a capacitor, so that it is difficult to make the varactor diode compatible with a high-frequency circuit (compatibility with the high-frequency operation).
Due to this, (BaxSr1−x) TiO3 (hereinafter abbreviated as BST) and other dielectric materials are proposed in order to constitute an element that is usable as a variable capacitance capacitor even in a high-frequency operation (See, for example, Japanese Unexamined Patent Publication JP-A 11-3839 (1999)).
The rate of change in the capacitance of a variable capacitance capacitor having a dielectric layer made of such a dielectric material becomes a function of the electric field intensity applied to the dielectric layer, so that the thickness of the dielectric layer used in a variable capacitance capacitor whose capacitance is changed by a low voltage must normally be several μm or less. In order to fabricate such a dielectric layer, methods such as the sputtering process, the sol-gel process and the CVD process are used. In order to fabricate variable capacitance capacitors comprising dielectric layers at low costs, in large quantities and stably, it is considered that the use of the sputtering process is effective. In order to fabricate the dielectric layer by the use of the sputtering process, it is a common practice to use, as the target, ceramics of the same composition as the dielectric to be obtained. In the sputtering process using such a ceramics target, it is a common practice that an RF sputtering apparatus is used, and that, as the sputtering atmosphere, a mixture resulting from adding an O2 gas into an Ar gas is used. Normally, in the case of forming a thin metal film, sputtering is performed in an atmosphere comprising only an Ar gas, but, in the case of using ceramics, the elimination of oxygen takes place at the time of film-forming deposition, as a result of which the oxygen concentration of the fabricated dielectric layer becomes less than that of the stoichiometric composition, and a large amount of lattice defects of oxygen are produced. In order to suppress the production of such lattice defects of oxygen, an O2 gas is introduced into the atmosphere at the time of sputtering. As a matter of fact, only by the alteration of the sputtering atmosphere by the introduction of an O2 gas, the formation of lattice defects of oxygen cannot be completely suppressed, and thus, in order to further reduce the lattice defects of oxygen, heat treatment is performed, after the sputtering, for a long time at a temperature higher than the substrate temperature at the time of sputtering (See, for example, Japanese Unexamined Patent Publication JP-A 9-31645 (1997)).
Further, a report is made on the method according to which, in order to suppress the deterioration in dielectric loss due to the lattice defects of oxygen, Mn or the like is added into the dielectric layer in addition to the heat treatment performed after the sputtering (See, for example, W. Chang, et al., Mat. Res. Soc. Symp. Proc. Vol. 541, (1999) 699)
However, the method of suppressing the deterioration in dielectric loss due to the lattice defects of oxygen or the production itself of lattice defects of oxygen is problematic in that the method requires a heat treatment extending over a long time after the sputtering and thus poor in efficiency from the viewpoint of a method of fabricating variable capacitance capacitors with an enhanced productivity; and thus, variable capacitance capacitors cannot be fabricated with low costs.
In order to use the thin film capacitor as a capacitor in a high-frequency circuit as mentioned above, the self-resonant frequency must be positioned at the side higher than the frequency used, the inductance must be small, and further, the loss of the electrodes must also be low. This is because, even in case that the resonance point is positioned at the side higher than the frequency at which the capacitor is used, the impedance resulting from the capacitor becomes small at a frequency in the vicinity of the resonance point, so that, in a capacitor with a large loss, the resistance component becomes predominant.
Thus, in order to reduce the loss due to the electrodes (for example, the loss of the high-frequency signal component when the capacitor is used in a high-frequency circuit), a metal that has a small resistivity must be used as the material of the electrodes, and the electrodes must be made as thick as possible.
However, at the steps of manufacturing a thin film capacitor, the electrodes are formed by the thin film technique and the photolithography method, so that the manufacturing steps become complicated. Further, there is the problem that, in case the electrodes are made too thick, the electrodes peel off from the surfaces to which they are to adhere.